RALEIGH, N.C. — The decline of marriage over the last generation has helped create an emerging voting bloc of unmarried women that is profoundly reshaping the American electorate to the advantage, recent elections suggest, of the Democratic Party. What is far from clear is whether Democrats will benefit in the midterm contests this fall.

With their Senate majority at stake in November, Democrats and allied groups are now stepping up an aggressive push to woo single women — young and old, highly educated and working class, never married, and divorced or widowed. This week they seized on the ruling by the Supreme Court’s conservative majority, five men, that family-owned corporations do not have to provide birth control in their insurance coverage, to buttress their arguments that Democrats better represent women’s interests.

But the challenge for Democrats is that many single women do not vote, especially in nonpresidential election years like this one. While voting declines across all groups in midterm contests for Congress and lower offices, the drop-off is steepest for minorities and unmarried women. The result is a turnout that is older, whiter and more conservative than in presidential years.

Half of all adult women over the age of 18 are unmarried — 56 million, up from 45 million in 2000 — and now account for one in four people of voting age. (Adult Hispanics eligible to vote, a group that gets more attention, number 25 million this year.) Single women have become Democrats’ most reliable supporters, behind African-Americans: In 2012, two-thirds of single women who voted supported President Obama. Among married women, a slim majority supported Mitt Romney.

“You have a group that’s growing in size, and becoming more politically concentrated in terms of the Democrats,” said Tom W. Smith, director of the General Social Survey at the National Opinion Research Center of the University of Chicago.

Single women, Democrats say, will determine whether they keep Senate seats in states including Alaska, Colorado, Iowa, Michigan and North Carolina — and with them, their Senate majority — and seize governorships in Florida, Pennsylvania and Wisconsin, among other states.

The party is using advanced data-gathering techniques to identify unmarried women, especially those who have voted in presidential elections but skipped midterms. By mail, online, phone and personal contact, Democrats and their allies are spreading the word about Republicans’ opposition in Washington — and state capitals like Raleigh — to pay equity, minimum wage and college-affordability legislation; abortion and contraception rights; Planned Parenthood; and education spending.

Nowhere is the courtship of unmarried women as intense as in North Carolina, where Senator Kay Hagan, a Democrat struggling for a second term, recently has shown gains even in a Republican poll. Midway through a recent Saturday of campaigning, she described her mobilization strategy: “Heels on the ground.”

Among those ground troops is Emma Akpan, an unmarried 28-year-old graduate of Duke Divinity School, who works to register voters but said she understood why so many single women are hard to reach. In an election without presidential candidates and the news media attention they draw, Ms. Akpan said, many women busy with jobs and perhaps children see no point in voting.

In the 2012 presidential election, 58 percent of single women voted. This fall that could slide to 39 percent, a one-third drop, according to projections from the nonpartisan Voter Participation Center, which for a decade has focused on unmarried women.

“A lot of these are single moms, they’re young, and young people don’t know when there’s an election,” said Representative Nancy Pelosi of California, the House Democratic leader, during a recent four-state bus tour to raise awareness. “It isn’t any lack of civic-mindedness. They’re just living their lives in a different way than, say, seniors are.”

In May, Stan Greenberg, a Democratic pollster, and James Carville, a party strategist, called on Democrats “to make major targeted efforts aimed at unmarried women.” They warned that support among them was down from early 2010, the previous midterm-election year, when the Tea Party’s rise powered a Republican romp.

The Democrats’ model is last year’s victory in the off-year election for Virginia governor. Terry McAuliffe, bolstered by groups like Planned Parenthood’s political advocacy arm, beat a conservative Republican officeholder after a campaign in which women were repeatedly reminded about his rival’s record against reproductive rights. In a race decided by just over two percentage points, Mr. McAuliffe won unmarried female voters by 42 points.

This year Democrats modified the McAuliffe model to emphasize pocketbook issues, too. While single women generally are socially liberal, “the issues they really care about are economic,” Mr. Greenberg said.

Personal economics help explain the difference in voting patterns between unmarried and married women, analysts say. Unmarried women, especially single mothers, have greater “economic vulnerability,” said Ruy Teixeira, a political demographer at the left-leaning Center for American Progress. “Married people are typically a bit more secure and have more buffers, so that tends to make them a bit more conservative.”

Democrats say one advantage they have this year, compared with 2010, is that they can cite Republicans’ voting records since taking power that year in the House and in states like North Carolina. “The policy issues that unmarried women care about are legitimately under attack,” said Kelly Ward, executive director of the Democratic Congressional Campaign Committee.

In response, Republican strategists are urging candidates to counter such talk of a Republican “war on women” by describing party policies as pro-family. Democrats “know if they can paint Republicans as meanspirited, that’s very helpful with women,” said Katie Packer Gage, a Republican consultant for the party’s effort to reach out to women. In a Twitter posting on Wednesday, her firm said, “Our party needs to take seriously the Democrats’ efforts to turn out single women.”

By then, however, a Fox News reporter had ignited a social-media furor by mocking the diverse bloc as Democrats’ “Beyoncé voters” — for the entertainer’s hit song “Single Ladies” — who depend on the government since they lack husbands.

The Democratic Congressional Campaign Committee calls its new voter-mobilization program Rosie, evoking Rosie the Riveter, for Re-engaging Our Sisters in Elections. Among outside groups, the Voter Participation Center has sent registration materials to single women in 24 states, including North Carolina, and will follow up through the fall.

Emily’s List and Planned Parenthood’s action fund are heavily engaged, and they will spend $3 million each on their top priority: Ms. Hagan’s race here against the Republican Thom Tillis. Of Planned Parenthood’s 140,000 members statewide, 50,000 joined since Republicans took power in 2011.

Mr. Tillis, as the state’s House speaker, is widely known for leading a run of conservative activism. The state has cut business taxes while reducing spending for education, unemployment benefits and Planned Parenthood. It has restricted abortion clinics, enacted voting limits, ended teacher tenure and rejected a federally funded Medicaid expansion. On the Tillis campaign website’s summary of his stand on issues, the symbol for jobs depicts a man’s dress shirt and tie.

“Voters are surely going to take a long look at what Thom’s done in the General Assembly,” said Daniel Keylin, his communications director. But “at the end of the day, this race is going to be a referendum on Kay Hagan and what she’s done in Washington” supporting a president who is unpopular here.

Since spring, the Republicans’ record has stoked a Moral Monday movement of weekly protests at the legislature, drawing a diverse crowd including labor, civil rights and women’s groups. “I have never been especially political,” said Jenny Spencer, 26, an unmarried lab technician, at one evening protest. “But in the last year or so I think it’s become increasingly important in North Carolina not just to have your own views but to really make a point of advocating them.”

Ms. Hagan, at a luncheon of Democratic women in Charlotte, promised “the biggest ground game we’ve seen in North Carolina for a U.S. Senate race,” adding, “It can only happen with your support.”

From there, Jackie Blair, a 34-year-old unmarried hotel operations manager, left to canvass door-to-door for Ms. Hagan. Raised a Republican in Nebraska, Ms. Blair said she switched because of Republicans’ stances on reproductive rights, health care and pay equity. At work, she said, she gets ribbed for encouraging other women to vote.

“They always say they will,” she said, “but I don’t know if they actually do.”

A version of this article appears in print on , on Page A1 of the New York edition with the headline: To Hold Senate, Democrats Rely on Single Women. Order Reprints | Today’s Paper | Subscribe